A single-cell type NOx sensor that includes a pair of electrodes on two sides of a solid electrolyte is disclosed in Patent Literature 1. A perovskite-type electrode is used as a detection electrode in the aforementioned NOx sensor. A perovskite-type electrode has selective absorptivity with respect to NOx. The aforementioned NOx sensor adopts a decomposition current that flows when NOx that was adsorbed on the detection electrode is decomposed as an output. The decomposition current has a correlation with the NOx concentration, and therefore the NOx concentration is detected by detecting the decomposition current.